This invention relates to liquid crystal displays and, in particular, to a system and method for controlling the drive voltage signals applied to a liquid crystal display.
Liquid crystal display (LCD) devices comprised of a layer of liquid crystal material sealed between first and second substrates with respective first and second sets of character-forming segments formed on opposing inner surfaces of the substrates are well known in the art. Such devices are passive or light-modulating in character and exhibit optical properties which change with the application of an electric field to selected portions thereof. Specifically, when no voltage is applied to the display segments or when the root mean square (rms) voltage is across the liquid crystal material is below an intrinsic threshold voltage (Vth) of the material, the display is in a light transparent state, giving it a clear appearance. This is hereafter called the OFF state. Upon application of a rms voltage which is greater than Vth to selected display segments, the liquid crystal molecules between the segments change from a light transparent to a light scattering state, thereby causing a dark portion to appear on the display. This is hereafter called the ON state. By selectively applying voltage signals to particular segments of the display, alpha and/or numeric information is displayable.
LCD devices employed commercially are generally of the "dynamic scattering" or "twisted nematic" types with the "twisted nematic" display being the most popular for calculator and watch applications. The "twisted nematic" structure is achieved by providing an alignment layer adjacent to the liquid crystal material which orients the director axes of the liquid crystal molecules adjacent to one major surface of the device at a predetermined angle, such as for example 90.degree., with respect to the director axes of the molecules at an opposite major surface of the device. Light energy is twisted according to the predetermined angle as it passes through the device. Polarizers are disposed adjacent to respective outer surfaces of the display substrates for linearly polarizing light passing through the display. The "twisted nematic" structure provides a clearly visible display at diverse viewing angles and under different ambient lighting conditions, although the display is typically not as bright or "jewel-like" in appearance as the "dynamic scattering" type of display.
Voltage drive systems for liquid crystal displays may be of the multiplexed or non-multiplexed type. In non-multiplexed systems, each segment is driven individually and is either fully on or fully off depending upon the value of the root mean square voltage across each segment. Multiplexed systems, on the other hand, employ time-sharing techniques whereby groups of segments are scanned rapidly and selected segments energized in sequence. Multiplexed displays are preferred because they require fewer drive lines and electrical interconnections, thereby reducing space, material requirements and cost. Although liquid crystal materials have the threshold characteristics required for multiplexing, the electro-optic response is angle dependent, temperature dependent and saturates as the optic axis of the sample gets near to alignment with the applied electric field, all of which tend to make multiplexing difficult. Nevertheless, one-third duty cycle multiplexing has been routinely achieved in commercial devices such as calculators, but higher levels of multiplexing are desirable to achieve cost savings, particularly as larger and more complex displays are needed.
It has been shown by Alt and Pleshko in their article entitled "Scanning Limitations of Liquid Crystal Displays", IEEE Transactions on Electron Devices, Vol. ED-21, No. 2, Feb. 1974, that the maximum number of display drive lines that can be effectively scanned or multiplexed, N.sub.MAX, is represented by the following equation. ##EQU1## Voff=root mean square drive voltage applied to the OFF segments of the display; and
Von=root mean square drive voltage applied to the ON segments of the display
Thus, the larger the ratio Voff/Von the greater the value of Nmax and the greater the number of lines that can be effectively multiplexed.
One previous technique for increasing the ratio Voff/Von was to modify the physical properties of the liquid crystal material to increase Voff/Von. This approach has not proved successful because of basic material limitations, particularly the control of elastic constants. Another approach is to take advantage of the fact that certain liquid crystal materials, particularly those which exhibit dynamic scattering properties, have a critical frequency above which the dielectric anisotropy changes from a positive value to a negative value. If a voltage signal comprised of a first component having a frequency below the critical frequency and a second component above the critical frequency is applied, the lower frequency signal will attempt to turn the liquid crystal molecules and the higher frequency signal will oppose that turning. By suitable choice of voltage levels for the two signals, Voff can be increased thereby increasing the ratio of Voff/Von. Disadvantages of this technique include the fact that the critical frequency is strongly dependent upon temperature, thereby making temperature compensation critical; power dissipation is increased considerably; the selection of liquid crystal materials is restricted to those with relatively low (2 KHz) critical frequencies; and unwanted effects such as reverse tilt are more likely to occur.
In addition, multiplexed display systems typically require three or more different voltage levels to achieve the desired level of multiplexing. This has the disadvantage of requiring additional components and circuitry such as voltage center taps and doubler circuitry.